Add an accessor for revisions for a Need

Pull the revisions out of the API response in the
Need model and make it available using attr_reader.

Before we assign it to the instance variable, turn
the hash into an OpenStruct using the method from
GdsApi::Response. Explicitly keep the changes field
as a hash, so we can do key/value iteration.
Add helper methods for field names and values

Add two methods to format field names and values
for a revision in a friendlier way, accounting for 
nil or empty values
Display a list of revisions in the need edit form

Render the collection of revisions from the Need API
at the end of the edit form. 

We display the changes between each revision, along
with the change time and the author, if available.